1. Field of the Invention
This invention relates to a technique of isolating element regions from one another by means of a V-shaped groove.
2. Description of the Related Art
FIGS. 1A to 1G show a conventional method of isolating element regions from one another by means of a V-shaped groove. First, as shown in FIGS. 1A and 1B, a V-shaped groove 2 is formed in a monocrystalline silicon substrate 1 of a predetermined crystal orientation by an anisotropic etching method (V-shaped groove etching). Then, as shown in FIG. 1C, a thermal oxide film 3 is formed on the entire region of the surface of the substrate 1 by a thermal oxidation method. Next, as shown in FIG. 1D, a polycrystalline silicon film 4 is deposited on the thermal oxide film 3. Thereafter, as shown in FIG. 1E, the substrate 1 is reversed. Then, as shown in FIG. 1F, the substrate 1 is ground so that the bottom portion of the V-shaped groove 2 is exposed. As a result, the substrate 1 is divided into a plurality of element regions 6 isolated from one another by the V-shaped groove 2. Thereafter, as shown in FIG. 1G, an element 7 is formed in each element region 6.
FIGS. 2A to 2H show another conventional method of isolating element regions from one another by means of a V-shaped groove. In this technique, two wafers are bonded and a V-shaped groove is formed in one of the wafers. First, as shown in FIG. 2A, a thermal oxide film 7 is formed on a monocrystalline silicon substrate 1B. Then, as shown in FIG. 2B, the monocrystalline silicon substrate 1A is bonded to the monocrystalline silicon substrate 1B (silicon wafer direct bonding process). The thermal oxide film 7 is sandwiched between the substrates 1A and 1B. Next, as shown in FIG. 2C, the substrate 1B, on which element regions are to be formed, is ground to a predetermined thickness. Thereafter, as shown in FIG. 2D, a V-shaped groove 2 is formed in the substrate 1B by an anisotropic etching method. Next, as shown in FIG. 2E, a thermal oxide film 3 is formed on the surface of the substrate 1B by a thermal oxidation method. Then, as shown in FIG. 2F, a polycrystalline silicon film 4 is formed on the thermal oxide film 3. Then, as shown in FIG. 2G, the polycrystalline silicon film 4 is ground so that the substrate 1B is exposed. As a result, the substrate 1B is divided into a plurality of element regions 6 isolated from one another by the V-shaped groove 2. Thereafter, as shown in FIG. 2H, an element 7 is formed in each element region 6.
In both of the above two methods, the V-shaped groove 2 is formed by means of an anisotropic etching method in the substrate of a predetermined crystal orientation. An alkali etching solution is used in the anisotropic etching method.
FIG. 3 is a diagram explaining a mechanism of forming a V-shaped groove in a substrate. Assuming that the surface orientation of the wafer is [100], the speed at which the wafer is etched in the direction perpendicular to the [111] plane (i.e., in the &lt;111&gt; direction) is much lower than those in the other directions. Thus, when the substrate 1 is etched by an anisotropic etching method, a V-shaped groove is formed in that portion which is not covered by a mask 10.
However, the anisotropic etching involves various errors. For example, as is shown in FIG. 4, an error .theta..sub.1 may be produced if the orientation flat 13 does not coincide with the actual crystal orientation 15. Further, for example, as is shown in FIG. 5, an error .theta..sub.2 may be produced if none of the edges of a photomask 17 for forming a mask pattern on the wafer is parallel to the orientation flat 13. If the above two errors simultaneously occur, as is shown in FIG. 6, the positional relationship between the mask pattern 18 on the wafer and the crystal orientation 15 has an error .theta.=.theta..sub.1 +.theta..sub.2. As a result, a V-shaped groove 19 in the substrate 1 extends under the mask 18, as shown in FIGS. 7 and 8. The distance between a corner of the mask 18 and the corresponding top edge of the groove is at most L.multidot.tan.theta. (L is the length of a edge of the mask 18). For example, assuming that the length L of an edge of the mask 18 is 1000 .mu.m and the error .theta. is 2.degree., the distance between a corner of the mask 18 and the corresponding top edge of the groove is at most 1000.times.tan2.pi. (2/360)=34 .mu.m.
The above conventional methods are disadvantageous in that the area of an element region is smaller than that of the mask 18 since the V-shaped groove extends under the mask 18. For example, if the mask 18 is a rectangle having sides L1 and L2, the distance between a corner of the mask and the corresponding top edge of the V-shaped groove is at most L1.multidot.tan.theta. or L2.multidot.tan.theta. (L represents the length of a side of the mask 18). Therefore, the element region 20 must be designed smaller than the mask region in consideration of the area of that region of the V-shaped groove which extends under the mask.